One common theme is the exploration of gender roles. Since Tanjiro is male in the original, making him female in fanfic allows for a look at how she would be treated in the demon - slaying world as a female. Another theme is new relationships. For example, how she might interact differently with other characters like Zenitsu or Inosuke as a female.
One common theme could be the relationship between Tanjiro and other characters, often with a romantic or emotional undertone. Since it's a 'lemon' fanfic, there might also be some elements of physical attraction and intimacy, but presented in a way that is related to the story and the characters' development.
Protection is a common theme. Tanjiro is always looking out for Nezuko, whether it's from other demons or humans who fear her. Another theme is loyalty. Nezuko's loyalty to Tanjiro is unshakable and this is often explored in fanfics.
One common theme is the growth of their relationship. It often starts from a simple acquaintance and develops into a deep emotional bond. For example, they might start as comrades in the fight against demons and gradually fall in love.
Love and friendship are very common themes. The fanfics often explore how the reader and Tanjiro develop a deep bond of love or friendship. Another common theme is adventure. Since Tanjiro is on a journey in the original story, many fanfics incorporate the reader into his adventures.
One common theme is friendship. They often start as comrades in the Demon Slayer Corps and develop a deep friendship. For example, they support each other in battles.
Yes, there are quite a few. Some focus on an alternate universe where they have to team up for some strange reason. For example, in one fanfic, they have to face a new common enemy that is even more powerful than Muzan. Tanjiro has to put aside his hatred for a while to deal with this new threat.
One great 'tanjiro x mitsuri fanfic' is 'A Bond Beyond the Demon Slayer Corps'. It really delves into their relationship, showing how their different personalities complement each other. Tanjiro's kind - hearted nature and Mitsuri's energetic and affectionate side create a really interesting dynamic in the story.